High temperature heat exchange using liquid metal



y 1967 J. P. MOGEE ETAL 3,321,009

HIGH TEMPERATURE HEAT EXCHANGE USING LIQUID METAL Filed June 1, 1965 A A H l J l INVENTORS JAMES I? M0655 NEIL H. 604755 A'rToRNEYs United States Patent 3,321,009 HIGH TEMPERATURE HEAT EXCHANGE USING LIQUID METAL James P. McGee and Neil H. Coates, Morgantown, W. Va., assignors to the United States of America as represented by the Secretary of the Interior Filed June 1, 1965, Ser. No. 460,605 6 Claims. (Cl. 1651) The invention herein described and claimed may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of royalties thereon or therefor.

This invention relates to high temperature exchange of thermal energy using an intermediate liquid metal exchange medium. In particular it relates to a heat exchange system wherein the heat source is maintained at a low pressure, such as ambient atmospheric pressure, and it is desired to transfer the energy to a process stream at a higher pressure.

In heat exchangers operating at high temperatures, presently available alloy tubes cannot be pressure stressed or they will fail. One means of avoiding these stresses is to maintain the pressures inside and outside the tubes at the same level. However, this is an expensive practice when products of combustion are used for direct heating since all of the combustion air must be compressed to the operating pressure. These high temperature heat exchangers are used to supply heat used in the gasification of coal and operate in the temperature range from about 1600 F. to about 20'00 F., usually above about 1700 F. At these temperatures it is necessary that the heat exchanger tubes be operated at a stress-free pressure differential between the outside and inside of the tubes. If any substantial difference in pressure is permitted to exist between the heating medium and the process stream or between the heating medium and the heat source, tube failure may result. In the exchange of thermal energy in such processes as a closed-cycle gas turbine or gasification of coal, it is desirable to heat a process stream at a pressure higher than ambient pressure or higher than the pressure of the heat source.

Accordingly, it is an object of this invention to provide methods and apparatus for exchanging heat between a lower pressure heat source and a higher pressure process stream. It is another object of this invention to provide a novel system for exchange of thermal energy employing an intermediate heat transfer fluid consisting of liquid metal. A further object is to increase the internal pressure within a liquid metal heat transfer medium between the heat source and the process stream to which thermal energy is transferred.

These and other objects and improvements of the instant invention will be seen more clearly in the description following and in the appended drawing in which the single figure is a diagrammatic flow sheet.

Referring to the drawing there is shown an apparatus and process wherein a heating zone or heat source 10 representing a first temperature zone in which a fuel supply 12, such as a fossil fuel, is mixed with combustion air from a low pressure source 14 in tubes 13, and tubes 15 containing a liquid metal at substantially the same pressure as combustion air source 14 are heated by the combustion products, which is withdrawn from the heating zone through conduit 16. The hot liquid metal at substantially the pressure of the heat source and at a temperature between about 1600 and 2000 F. passes to a circulating system through conduit 18. The liquid head of the molten metal causes an increase in the internal pressure of the liquid metal progressively along the path to a lower point in the system. In order to compensate for the increased internal pressure along the circulating system and to maintain the pressure outside the conduit substantially equal to that inside, a series of pressurized segments 20, 21, 22, 23, 24, 25, 26 and 27 are provided along the flow path of the liquid metal. These segments are maintained under a pressure increased incrementally in a direction from the heat source to the process heat exchanger 30. This pressurization may be accomplished in a number of ways, one of which would be to supply an inert gas to the segments. The gas is fed to the lowermost pressure segment 27 at its high pressure level via conduit 28 and pressure regulators 29 disposed between the successive segments lower the pressure incrementally. A back-pressure regulator 35 located on the outlet 37 (vent) of the last pressurized segment 20 controls the inert gas flow through the segments. A variety of gases, such as helium, argon, or nitrogen can be used.

The pressure of the second temperature: zone at heat exchanger 30 is determined by the specific gravity of the intermediate liquid metal being used and by the difference in elevation AH between the heating zone 10 and the heat exchanger 30. In order to avoid pressure-stressing the liquid metal heat transfer tubing 31 in heat exchanger 30, the process stream 32 to be heated by the liquid metal in tubes 33 must be maintained at a pressure substantially equal to the pressure of the heating zone plus the added pressure of the liquid metal fluid head. If liquid lead is used as the intermediate material, a pressure differential of about 30 atmospheres would result for feet of elevational difference between the first temperature zone 10 and the second zone 30. The heated process stream is withdrawn from the system by conduit 34.

The cooled liquid metal is passed from the heat exchanger 30 via conduit 36 through the incrementally pressurized segments, which may be separate from the segments surrounding conduit 18, or may be integral as shown in the drawing. Located in the return or recirculating conduit 36 is a pumping means 40, such as an electromagnetic pump. Recent advances in cryogenic magnets make this type of pump most desirable for high temperature fluids such as liquid metals.

The heat source for heating zone 10 may be high temperature heat from the atmospheric combustion of a fossil fuel, or some other source of thermal energy such as a nuclear reactor may be used. Liquid metal intermediate heat transfer materials are known for use in the latter process, and prior art workers have successfully used liquid sodium, potassium, etc., for cooling nuclear reactors with subsequent transfer of the heat to a process stream and recirculation of the liquid medium. These metals are used in the nuclear system for their low crosssection for neutron absorption; however, when used above 1100' F. considerable difficulty is encountered. Lead or its alloys, which are much less reactive than the alkali metals, may be used when the heat source is a fossil fuel, such as coal, gas or oil.

Any process stream to be heated to a high temperature may be the input to the heat exchanger 30. One specific example of the coal gasification process is an endothermic method in which coal and steam are reacted at 1700 F. or higher and at a pressure substantially equal to the internal liquid metal pressure to produce synthesis gas. Combustion of fossil fuel at atmospheric pressure heats a recirculated metal intermediate heat exchange medium. The hot liquid metal transfers its heat to the steam-coal mixture at a higher pressure, determined by the specific gravity and fluid head of the liquid metal transfer medium.

Another example is the closed-cycle gas turbine wherein coal burned at ambient pressure heats the liquid metal, which in turn transfers the thermal energy to a high pressure gas serving as the turbine working fluid.

In another embodiment of the invention, not shown, the exchange of heat to the process stream may be effected by direct contact between the intermediate liquid metal and a process fluid. Such method may be carried out in a heat exchanger comprising a pressure vessel having inlet and outlet conduit for the process stream and for the liquid metal stream and having means for mixing the phases. This embodiment of the invention requires that there be no detrimental reaction between the streams and preferably is carried out in a countercurrent contacting apparatus.

The invention has been illustrated by specific examples but there is no intent to limit the invention to the specific details so disclosed in the description and drawing, except insofar as set out in the following claims.

What is claimed is:

1. A heat transfer method comprising the steps of '(a) circulating liquid metal in a heating zone maintained at a temperature above 1600 F. and at ambient pressure,

(b) passing the heated liquid metal to a heat exchanger at a lower elevation,

'( c) introducing a process stream to be heated into the heat exchanger at a pressure substantially equal to the internal pressure of the liquid metal in the heat exchanger, and

(d) recirculating the liquid metal to the heating zone.

2. The method of claim 1 wherein the liquid metal is circulating in a closed system.

3. The method of claim 1 wherein liquid metal circulation conduit is incrementally :pressurized to maintain the outside conduit pressure substantially equal to the inside conduit pressure.

4. A heat transfer method comprising the steps of (a) circulating liquid metal in a heating zone maintained at a high temperature and at a low pressure,

(b) passing the heated liquid metal to a heat exchanger at a lower elevation, thereby increasing the internal pressure of the liquid metal to a higher pressure,

(c) introducing a process stream to be heated into the heat exchanger at a pressure substantially equal to the higher pressure, and

(d) recirculating the liquid metal to the heating zone.

5. A heat exchange apparatus comprising a first temperature zone having means for heating an intermediate liquid metal exchange medium at a pressure substantially equal to the ambient pressure of said first temperature zone;

means for circulating said liquid metal to a second temperature zone comprising an incrementally pressurized conduit, having a pressure increasing from the first temperature zone to the second temperature zone;

means for introducing material to be heated in said second temperature zone at a pressure substantially equal to the ambient pressure plus the pressure of the liquid metal; and

means for returning the liquid metal to the first temperature zone comprising an incrementally pressurized conduit, having a pressure increasing from the first temperature zone to the second temperature zone.

6. A heat exchange system according to claim 5 wherein the liquid metal circulating and returning means contains an electromagnetic pump.

References Cited by the Examiner UNITED STATES PATENTS 2,971,746 2/1961 Bell -107 ROBERT A. OLEARY, Primary Examiner.

CHARLES SUKALO, Examiner. 

1. A HEAT TRANSFER METHOD COMPRISING THE STEPS OF (A) CIRCULATING LIQUID METAL IN A HEATING ZONE MAINTAINED AT A TEMPERATURE ABOVE 1600*F. AND AT AMBIENT PRESSURE, (B) PASSING THE HEATED LIQUID METAL TO A HEAT EXCHANGER AT A LOWER ELEVATION, 